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126                         M1TALLUEGY OF IRON AND STEEL.
most of the melting operation, escape to the stack, showing a dull red or a full red temperature.
The space occupied by the air and gas checkers combined should he at least 50 cubic feet per ton of steel in the furnace, while to get, the best results this figure should be at least doubled. In other words, in a 50-ton furnace the checker bricks in each chamber' should occupy at least 2500 cubic feet, which is equivalent to a space 16'xl6'xlO', while, if they occupy a space 20'x20'xl2', there will be a saving in fuel. These dimensions do not include the space below the bricks to give draft area for the gases, nor. the space above the bricks to allow the flame to spread over the whole surface of the chamber."
In the 40-ton Steelton furnace, in Fig. VIII-B, the volume occupied by the air checkers is about 45 feet per ton; the gas chamber is less, so that the total is from 65 to 70 feet for both chambers. The double passage, however, allows a better absorption than would be given by the same volume in one mass. In the 50-ton Steelton furnace in Fig. VIIL-C the total checker volume on one end is about 100 feet; in the 30-ton Donawitz furnace in Fig. VIII-D about 110 feet; in the 50-ton Duquesne furnace in Fig. VIII-E about 55 feet, and in the 50-ton Sharon furnace in Fig. VIII-F about 90 feet.
In another open-hearth plant the gas checkers on each end occupied 17 cubic feet per ton of steel and the air checkers 32 cubic feet. The products of combustion' passing to the chimney from this furnace were red hot during a portion of the operation.
The information just given is by no means sufficient in stating merely the space occupied by the bricks, for it is fully as important to know the space left between them for the passage of the gases. The area of these channels must be far in excess of the area of the ports or of the flue leading to the chimney, since the friction caused by the small passages will retard the flow of gases, and this retardation will increase continually during the running of the furnace owing to the deposits of dust in these passages, decreasing the size of the orifices -and forming a rough surface for the current to pass over. For this reason the sum of the area of all the passages between the bricks must be several times as great as the size of the flues and ports. The area between the bricks will in great measure determine the life of the checker bricks, for these bricks must be